Numerous processes are known in the art for the production of an ultra-high purity oxygen product stream by using cryogenic distillation; among these are the following:
U.S. Pat. No. 3,363,427 discloses a process for the production of ultra-high purity oxygen from a commercial grade oxygen stream, which typically has an oxygen concentration of about 99.5-99.8 vol %, a small amount of argon as a light impurity and small quanities of heavier impurities consisting of a variety of hydrocarbons (mainly methane), krypton and xenon. In the process, hydrocarbons are either removed by combustion in a catalytic chamber or as purge liquid from an auxiliary distillation column. When a catalytic combustion unit is not used, multiple distillation columns are used with various heat exchangers and reboiler/condensers to effectuate the separation. In this operating mode, refrigeration to the system is provided by either importing liquid nitrogen from an external source or using a nitrogen stream from the air separation unit that is recycled back to the air separation unit, thus transferring refrigeration from one point to another. This catalytic combustion option requires an additional compressor and heat exchangers.
U.S. Pat. No. 4,560,397 discloses a process to produce ultra-high purity oxygen and a high pressure nitrogen by cryogenic distillation of air. In the process, the feed air is fractionated in a high pressure column producing a nitrogen product stream, which is removed from the top of the high pressure column, and a crude liquid oxygen stream, which is removed from the bottom of the high pressure column. This crude liquid oxygen stream is laden with all the heavy impurities contained in the feed air and also contains a majority of the argon contained in the feed air. A portion of this crude liquid oxygen stream is distilled in a secondary lower pressure column to produce a so called ultra-high purity oxygen. Since all the heavy impurities will travel with the oxygen downward in this secondary column, it is impossible to produce a liquid oxygen product with trace low concentrations of impurities directly from this column. To overcome this problem, a gaseous oxygen product is removed at a point at least one equilibrium stage above the reboiler/condenser of this secondary column. Since, however, this vapor stream is in equilibrium with a liquid stream with high concentrations of heavies it is impossible to reduce the concentration of heavy impurities to the desired levels. For example, referencing the results cited in this patent, the concentration of methane in the so-called ultra-high purity oxygen is 8 vppm and of krypton is 1.3 vppm. By the ultra-high purity oxygen standards required specifically for electronic industry, these concentrations would be considered high; the typical hydrocarbon content of ultra-high purity oxygen for the electronic industry is less than 1 vppm.
U.S. Pat. No. 4,755,202 discloses a process to produce ultra-high purity oxygen from an air separation unit using double column cycle. In this process, an enriched oxygen containing stream (oxygen concentration range from 90.0 to 99.9%) is withdrawn from the bottom of the lower pressure column and is fed to a counter-current absorption column. In the absorption column, the ascending enriched oxygen containing stream is cleaned of heavier components by a descending liquid stream. A hydrocarbon-lean enriched oxygen containing stream is removed from the top of the absorption column and is subsequently condensed. A portion of this condensed hydrocarbon-lean stream is recyled as reflux to the absorption column, while the other portion is sent to a stripping column. In the stripping column, the descending hydrocarbon-lean liquid stream is stripped of the light components, such as argon, to produce an ultra-high purity liquid oxygen product at the bottom. A portion of the ultra-high purity liquid oxygen is reboiled to provide a vapor stream for the stripping column. This vapor stream is removed from the top of the stripper column and is recovered as a secondary product. In essence, this process has two undesirable features. The first is that by using a feed oxygen stream from the bottom of the low pressure column which is contaminated with both light and heavy impurities, two distillation columns are required to perform the separation (an absorption column and a stripping column). The second is that the process generates an oxygen containing vapor stream at the top of the stripping column which has an increased argon concentration; it is usually undesirable to have secondary oxygen product stream with decreased oxygen content.
U.S. Pat. No. 4,869,741 discloses a process to produce ultra-high purity oxygen. In the process, a liquid oxygen containing heavy and light contaminants is used as the feed stream. In the process, two distillation columns, three reboiler/condensers and a compressor on the recirculating nitrogen stream along with a main heat exchanger are used to effectuate the separation.